52 PEOF. p. M. DUNCAN AND ME. W. P. SLADEN ON 



(see P]. II. fig. 3) all the poriferous plates are simple and entire 

 — that is to say, primary plates which extend from the median 

 suture of the area to its outer margin, each with a single pair of 

 pores. At a very short distance from the apex — on the fifth or 

 seventh plate from the radial (ocular) plate in the case of A. 

 stellata — there is an unmistakable tendency to form binary com- 

 pound plates, which manifests itself in the greater increase of the 

 plate in question than in its immediate neighbours of the same 

 column. This enlargement of tlie plate takes place at its inward 

 end, in such a way as to environ the whole of the inner end of the 

 next aboral poriferous plate, thus intercepting the entry of that 

 plate into the median suture, and preventing its further growth in 

 that direction, causing it to remain a small or demi-plate hence- 

 forward. It may also be seen that one, or sometimes two, simple 

 primary plates intervene between each of the succeeding binary 

 compounds thus formed. These ultimately become the adoral 

 small or demi-plates of the ternary compounds, consequent on an 

 adoral encroachment of the main primary poriferous plate, 

 similar to its aboral increase just noted. It will further be ob- 

 served that the increased primary poriferous plate in one column 

 of the ambulacrum is opposite to a small poriferous plate in the 

 adjacent column, the growth of which it appears to have pre- 

 vented, and thus perhaps determines the reason why the small 

 primary plate (at this period still an entire one) which underlies, 

 or is adoral to, the binary compound plate in the adjacent 

 column, ultimately becomes the adoral member or small demi- 

 plate in the mature ternary compound. This alternate in- 

 crease and debarred growth is necessarily reciprocal in the two 

 columns. It is not till near the ambitus that the true fully -formed 

 ternary compounds, previously described, are met with. In a 

 small test with a diameter of 33 millim., the 23rd, 24th, and 25th 

 poriferous plates constitute the first true typical ternary com- 

 pound ambulacral plate ; though, critically considered, the two 

 preceding triplets might essentially be almost ranked as such. 



Jjo. A. punctulata t\e fi.v^t true ternary compound ambulacral 

 plates are as near to the ambitus as in A. stellata ; the 23rd, 24th, 

 and 25th primary poriferous plates, or even the 26th, 27th, and 

 28th from the apex, constituting the first typical triplet. 



In A. pustulosa the ternary plates have the appearance of being 

 somewhat further removed from the ambitus, for although when 

 counted from the apex the position of the first typical triplet 



